When should Group B Streptococcus (GBS) testing be performed prenatally?

Group B Streptococcus Screening in Pregnancy

All pregnant women should be screened for Group B Streptococcus (GBS) colonization at 35-37 weeks' gestation through vaginal-rectal swab culture. 1, 2

Timing of GBS Screening

The optimal timing for GBS screening is critical for effective prevention of neonatal GBS disease:

• 35-37 weeks' gestation: CDC and ACOG recommend universal screening during this window 1, 2

  • This timing provides the best sensitivity and specificity for detecting women who will remain colonized at delivery
  • Screening too early is not recommended as GBS colonization can be transient, intermittent, or chronic

• ACOG updated recommendation: 36 0/7 to 37 6/7 weeks of gestation 3

  • This is a slight modification to the CDC guidelines but falls within the same general timeframe

Proper Specimen Collection Technique

The accuracy of GBS screening depends on proper collection technique:

• Collection sites: Lower vagina (vaginal introitus) followed by rectum (through anal sphincter) 1, 2
• Method: Use the same swab or two different swabs for both sites
• Avoid: Do not use a speculum for collection
• Unacceptable alternatives: Cervical, perianal, perirectal, or perineal specimens are not acceptable 2

Special Circumstances for GBS Testing

Women Who Do Not Need Routine Screening

Some women should automatically receive intrapartum antibiotic prophylaxis without screening:

• Women with GBS bacteriuria during any trimester of the current pregnancy 1, 2
• Women who previously delivered an infant with invasive GBS disease 1, 2

Preterm Labor or Preterm Premature Rupture of Membranes

For women presenting with signs of preterm labor or PPROM:

• Obtain GBS culture at hospital admission if not done within the previous 5 weeks 1
• Start GBS prophylaxis immediately for women with:
  • Unknown GBS status at admission
  • Positive GBS screen within the preceding 5 weeks 1
• Discontinue antibiotics if not in true labor (for preterm labor cases) 1
• Repeat screening at 35-37 weeks if the patient has not yet delivered 1

Clinical Implications and Importance

GBS screening is crucial for preventing neonatal morbidity and mortality:

• GBS is the leading cause of newborn infection 3
• Maternal colonization is the primary risk factor for neonatal early-onset disease 2
• Colonized women are >25 times more likely to deliver infants with early-onset GBS disease 2
• Approximately 10-30% of pregnant women are colonized with GBS 4

Common Pitfalls to Avoid

1. Improper specimen collection: Failure to sample both vaginal and rectal sites reduces detection sensitivity
2. Incorrect timing: Screening too early or too late reduces accuracy
3. Oral antibiotic treatment during pregnancy: This is ineffective in eliminating GBS carriage and preventing neonatal disease 1, 2
4. Relying on previous pregnancy results: GBS status in a previous pregnancy does not predict current status unless there was a previous GBS-affected infant 1
5. Missing high-risk women: Women with planned cesarean deliveries should still undergo screening in case labor begins before the scheduled procedure 1

Emerging Technologies

While standard culture remains the recommended method, newer rapid testing methods show promise:

• PCR-based tests: Have shown higher sensitivity (>90%) compared to standard culture 4, 5, 6
• Point-of-care testing: May be valuable for women presenting in labor without prior screening results 6

However, these newer methods are not yet universally recommended to replace standard culture-based screening at 35-37 weeks.